What is the difference between =, == and -eq in shell scripting?
Is there any difference between the following?
[ $a = $b ][ $a == $b ][ $a -eq $b ]Is it simply that = and == are only used when the variables contain numbers?
Best Answer
= and == are for string comparisons-eq is for numeric comparisons-eq is in the same family as -lt, -le, -gt, -ge, and -ne
== is specific to bash (not present in sh (Bourne shell), ...). Using POSIX = is preferred for compatibility. In bash the two are equivalent, and in sh = is the only one that will work.
$ a=foo$ [ "$a" = foo ]; echo "$?" # POSIX sh0$ [ "$a" == foo ]; echo "$?" # bash-specific0$ [ "$a" -eq foo ]; echo "$?" # wrong-bash: [: foo: integer expression expected2(Note: make sure to quote the variable expansions. Do not leave out the double-quotes above.)
If you're writing a #!/bin/bash script then I recommend using [[ instead. The double square-brackets [[...]] form has more features, a more natural syntax, and fewer gotchas that will trip you up. For example, double quotes are no longer required around $a:
$ [[ $a == foo ]]; echo "$?" # bash-specific0See also:
- What's the difference between [ and [[ in Bash?
It depends on the Test Construct around the operator. Your options are double parentheses, double brackets, single brackets, or test.
If you use ((…)), you are testing arithmetic equality with == as in C:
$ (( 1==1 )); echo $?0$ (( 1==2 )); echo $?1(Note: 0 means true in the Unix sense and a failed test results in a non-zero number.)
Using -eq inside of double parentheses is a syntax error.
If you are using […] (or single brackets) or [[…]] (or double brackets), or test you can use one of -eq, -ne, -lt, -le, -gt, or -ge as an arithmetic comparison.
$ [ 1 -eq 1 ]; echo $?0$ [ 1 -eq 2 ]; echo $?1$ test 1 -eq 1; echo $?0The == inside of single or double brackets (or the test command) is one of the string comparison operators:
$ [[ "abc" == "abc" ]]; echo $?0$ [[ "abc" == "ABC" ]]; echo $?1As a string operator, = is equivalent to ==. Also, note the whitespace around = or ==: it’s required.
While you can do [[ 1 == 1 ]] or [[ $(( 1+1 )) == 2 ]] it is testing the string equality — not the arithmetic equality.
So -eq produces the result probably expected that the integer value of 1+1 is equal to 2 even though the right-hand side is a string and has a trailing space:
$ [[ $(( 1+1 )) -eq "2 " ]]; echo $?0While a string comparison of the same picks up the trailing space and therefore the string comparison fails:
$ [[ $(( 1+1 )) == "2 " ]]; echo $?1And a mistaken string comparison can produce a completely wrong answer. 10 is lexicographically less than 2, so a string comparison returns true or 0. So many are bitten by this bug:
$ [[ 10 < 2 ]]; echo $?0The correct test for 10 being arithmetically less than 2 is this:
$ [[ 10 -lt 2 ]]; echo $?1In comments, there is a question about the technical reason why using the integer -eq on strings returns true for strings that are not the same:
$ [[ "yes" -eq "no" ]]; echo $?0The reason is that Bash is untyped. The -eq causes the strings to be interpreted as integers if possible including base conversion:
$ [[ "0x10" -eq 16 ]]; echo $?0$ [[ "010" -eq 8 ]]; echo $?0$ [[ "100" -eq 100 ]]; echo $?0And 0 if Bash thinks it is just a string:
$ [[ "yes" -eq 0 ]]; echo $?0$ [[ "yes" -eq 1 ]]; echo $?1So [[ "yes" -eq "no" ]] is equivalent to [[ 0 -eq 0 ]]
Last note: Many of the Bash specific extensions to the Test Constructs are not POSIX and therefore may fail in other shells. Other shells generally do not support [[...]] and ((...)) or ==.
== is a bash-specific alias for = and it performs a string (lexical) comparison instead of a numeric comparison. eq being a numeric comparison of course.
Finally, I usually prefer to use the form if [ "$a" == "$b" ]
Several answers show dangerous examples. The OP's example, [ $a == $b ], specifically used unquoted variable substitution (as of the October 2017 edit). For [...] that is safe for string equality.
But if you're going to enumerate alternatives like [[...]], you must inform also that the right-hand-side must be quoted. If not quoted, it is a pattern match! (From the Bash man page: "Any part of the pattern may be quoted to force it to be matched as a string.").
Here in Bash, the two statements yielding "yes" are pattern matching, other three are string equality:
$ rht="A*"$ lft="AB"$ [ $lft = $rht ] && echo yes$ [ $lft == $rht ] && echo yes$ [[ $lft = $rht ]] && echo yesyes$ [[ $lft == $rht ]] && echo yesyes$ [[ $lft == "$rht" ]] && echo yes$